In silicon P-N junction photodiodes, photons of incident light penetrate the Si surface and electron-hole pairs are generated when a photon is absorbed. The light intensity decreases exponentially from the incident surface as photons are absorbed. The penetration depth into the Si is dependent on the wavelength of the incident light. The absorption length (distance for light intensity to decrease to 1/e of initial intensity at surface) varies from 3 microns at 650 nm to 25 microns at 900 nm. Any generated minority carriers (electrons in P-type material or holes in N-type material) which diffuse to and crosses P-N junction results in photodiode current signal.
A conventional silicon P-N junction photodiode is made by the formation of N-type or P-type layer on P-type or N-type substrate material, respectively, by diffusion, implantation, or epitaxial deposition. Some examples of conventional photodiodes are shown in FIGS. 1a-1f. Surface leakage current exists in all such photodiodes and is proportional to junction surface periphery.
Many photodector applications include acquisition of information about spectral distribution of incident light or about light available in a certain wavelength range. The conventional measurement approach is to use optical filters over photodiodes. For example, Texas Instruments manufactures a photodetector for a camera. That photodetector has two adjacent conventional junction photodiodes to determine spectral distribution information about incident light for camera exposure control. However, such detectors suffer from surface junction leakage current losses, require twice the surface area of a single photodiode (and thus further surface leakage) and have an added external element (optical filter 8) that is not a part of the normal fabrication process for integrated circuits. So, it is desired to have a photodiode that can discriminate between or among different frequencies of light, one that has less surface junction leakage, one that eliminates the use of a separate, optical filter, and one capable of integration with the manufacture of other electronic circuits.